Field of the Invention
The invention relates to a method for open-loop/closed-loop control of the current converter valves of at least two load-guided parallel oscillating circuit inverters being supplied from a common direct current or direct voltage source and connected in series on the DC side, each having one induction furnace with a compensating capacitor, wherein by ignition of two diagonally opposed current converter valves at a time in one inverter, a flow of current from the direct current source or direct voltage source with a series-connected choke is established through the parallel oscillating circuit connected to that inverter.
A parallel oscillating circuit inverter for inductive heating is known from Siemens-Zeitschrift [Siemens Journal] 45 (1971), No, 9, pp, 601-606. That involves an intermediate circuit frequency converter circuit with impressed current in the intermediate circuit, The direct current generated by the power rectifier from an alternating mains voltage is converted into a medium-frequency alternating current in the load-side inverter. The load includes an inductor that inductively transmits the operative power to the product (such as molten metal) to be heated. In order to cover the high reactive power demand, the inductor is expanded with the aid of one or more compensating capacitors to make a parallel oscillating circuit. In the load-guided inverter, the impressed intermediate circuit current is alternatingly switched to the load at the clock rate of the operating frequency, through diagonally opposed valve branches, and generates a virtually sinusoidal voltage in the load. The operating frequency that comes to be established depends directly on the natural frequency of the load circuit, because of the load clocking.
In a system including a plurality of induction furnaces, each with its own parallel oscillating circuit inverters, it is usual to connect each parallel oscillating circuit inverter to its own direct current source, or in other words its own rectifier, with a direct current intermediate circuit, which is very expensive.